1. Field of the Invention
The present invention relates to a technique for more accurately determining a ring center in a case wherein correction (ring artifact correction) is performed to remove, for example, a concentric artifact (ring artifact) occurring in an image captured by using an X-ray diagnostic apparatus.
2. Description of the Related Art
A ring artifact is well known in the field of X-ray computed tomographic apparatuses, and is said to occur due to a small sensitivity error of a detector element. This artifact is very low in density but poses a problem especially when soft tissue or the like is to be observed.
Recently, there has been an increasing need to three-dimensionally image soft tissue and observe the resultant image by using an X-ray diagnostic apparatus including a two-dimensional detector. Such three-dimensional imaging is generally performed by using a system comprising a rotating gantry such as an almost C-shaped arm instead of a rigid rotating gantry like that of an X-ray computed tomographic apparatus. Such a system executes radiography for X-ray images upon stopping the almost C-shaped arm (rotational radiography system), on which an X-ray generating unit and an X-ray detecting unit are mounted, at a desired position by, for example, translating and rotating the C-shaped arm. For this reason, a mechanical system comprising the support portion, driving portion, and the like of the almost C-shaped arm suffers from vibrations or deflection accompanying movement or stoppage, backlash due to long-time use, the movement of the center of gravity after rotation through a given angle or more, and the like. Such variations and the like cause wobbling of the almost C-shaped arm around its rotation center, thus causing a deterioration in image quality.
In order to solve this problem, a conventional X-ray diagnostic apparatus radiographs a phantom of a known structure, derives the relative positional relationship between the phantom and an obtained projection image, and corrects the influences of vibrations and the like on the image by using the relationship.
When ring artifact correction is to be performed, it is necessary to identify the position of the center of a ring artifact (ring center) on an image. This ring center corresponds to the rotation center of a rotational radiography system. A conventional X-ray diagnostic apparatus having an almost C-shaped arm defines the system of the reconstruction coordinates with reference to a phantom center used for the acquisition of correction data for removing the influences of vibrations and the like. Consequently the position of the ring center depends on the position of the phantom.
In the conventional X-ray diagnostic apparatus, since the positions of the rotation center (that is the position of the ring center) is identified by manual operation on reconstruction images, a problem arises in terms of accuracy. In addition, every time correction data for removing the influences of vibrations and the like is updated, it is necessary to identify the position of a ring center. This operation is very cumbersome and is not practical.